This invention relates to a technique for calibrating the ionosphere in respect to columnar electron content on a realtime universally applicable basis, and to application of that calibrating technique to surveillance of large-scale geophysical events which modify the ionosphere.
The ionosphere has been extensively explored in nonrealtime. The earliest technique involved the Ionosonde utilizing a pulsed transmitter that sends radio signals vertically upward while slowly sweeping the radio frequency.
Pulses reflected by the ionosphere are received and recorded; the elapsed time between pulsed transmission and reception can be interpreted as an apparent distance to the point of reflection. This sounds only the bottom side of the ionosphere. The resulting presentation is a curve of apparent height vs. frequency. This technique is still profitably and extensively used today. In recent years, many rockets have flown through the ionosphere to record data, and satellites have orbited above the ionosphere for similar purposes. An ionosonde in a satellite has been utilized to explore the topside of the ionosphere (that portion above the region of maximum electron concentration).
The Ionosonde technique applied at ground stations showed that the ionosphere is structured. It was first thought that discrete layers were involved, commonly referred to as D, E, F.sub.1, F.sub.2. However, rocket measurements have shown that the "layers" merge with one another to such an extent that they are now normally referred to as regions. A vertically incident radio wave is reflected from the ionosphere at that level where natural frequency of the plasma equals the radio frequency. This procedure makes it possible to identify the electron concentration at the point of reflection in terms of the radio frequency according to the relation EQU n.sub.e =1.24.times.10.sup.10 f.sup.2 ( 1)
where n.sub.e is the electron concentration per cubic meter and f is the radio frequency in megahertz.
The D region is the lowest ionospheric region extending approximately from 60 to 85 kilometers. The D region is susceptible to disturbance particularly due to x-ray bursts. The sudden ionospheric disturbances (SID's) occur over the daylight hemisphere with some solar flares; it can be shown that SID's also occur because of acoustic pressure, atomic blasts (including underground), volcanic eruptions, earthquakes which cause acoustic percussion, and large tropospheric storms.
VHF Faraday rotation has been used to estimate the charged particle count of the ionosphere, but this Faraday effect depends upon Earth's magnetic field and no measurable effect exists for electrons out of the magnetic field influence. Therefore, this technique cannot inherently measure the total electron content without modeling associated parameters.
A surprisingly large number of applications for accurate ionospheric measurements exist. The ionosphere represents a significant error source for high precision radio metric tracking data. For example, 1.times.10.sup.17 el/m.sup.2 zenith columnar content during night hours results in 12 cm of baseline length increase on an Astronomical Radio Interferometric Earth Surveying (ARIES) geodetic measurement at S-band on a 336 km distance. Spacecraft tracking at S-band and to a lesser degree at X-band by a factor of 13, also suffers from errors caused by free electrons in the radio path which are substantially the Earth's ionosphere. The time variable ionospheric electron columnar content in the tracking station to spacecraft line of sight causes variations which are highly correlated with the Doppler effects caused by changes in spacecraft right ascension and declination positions.
Less prosaic applications also exist because of the effect of troposphere to ionosphere dynamic coupling. Disturbances in the troposphere propagate into the ionosphere and can cause propagating ionospheric waves. Speculations exist concerning impulsive input in the troposphere which has manifestation in the ionosphere. For example, a tsunami event in the ocean causes an acoustic impusles which propagates into the ionosphere. The March 1980 Mount St. Helens eruption was very clearly evident in VHF Faraday rotation ionospheric monitoring at Goldstone, Calif. That Faraday rotation instrumentation has a sensitivity of approximately 3.times.10 .sup.15 el/m.sup.2 on 1 minute samples and an ambiguity of 4.times.10 .sup.16 el/m.sup.2. Other impulsive events of interest are nuclear explosions, especially underground explosions in which seismic hiding methods might be employed to confuse seismic detection of such events. If conducted underground, the explosive impulse could be transmitted to the surface and result in a tropospheric impulse which may be detectable as an ionospheric columnar electron content modification.
More direct influences in the ionosphere will also be detectable such as particle interactions, principally solar induced, but perhaps the result of ionospheric heating by input electromagnetic energy or disturbances caused by hot gases expelled by missiles during their boost phase or perhaps the testing of particle beam weapons.
Large scale weather systems in the troposphere and stratosphere probably also effect ionospheric patterns providing yet another opportunity to study global meteorological processes. Combining these ionospheric observations with the direct range and Doppler measurements from high precision geodetic receivers will yield small scale variations along the line of sight which are caused by tropospheric index of refraction changes. The water vapor and dry tropospheric components are responsible for these index changes which can be detected with millimeter path equivalent sensitivity once ionospheric effects have been removed.
It is thus evident that if the ionosphere is continually calibrated, any sudden change in the ionosphere will signal a terrestrial event within seconds. It is important to detect some events in realtime, such as a tsunami event in order to alert those in the path of these destructive sea waves, or the launching of an intercontinental missile in order to launch an intercepting missile. Other events, such as an underground atomic explosion of an experimental nature may not be as critical, but the ability to detect it in realtime would still be important.
The problem then is one of providing a technique for continual calibration of the ionosphere in realtime on at least a regional scale, but preferably on a global scale. This capability has not heretofore been available. Since the localized ionosphere is modified by terrestrial events because of troposphere acoustic coupling and direct atomic interaction, such as is due to hot gases expelled from missiles in their boost phase or plasma clouds in front of ballistic reentry vehicles or particle-beam impingng on the ionosphere, a plurality of ionosphere surveillance stations could then be clustered about any strategic area to provide passive continuous monitoring of that area for military applications as well as civil, commercial and geodetic applications while being compatible with existing active and passive local surveillance systems.